1. Field of the Invention
The present invention relates to wire ropes used for heavy duty hoisting applications. The wire rope of the invention finds particular utility in large excavating equipment used, for example, in mining. Wire ropes are employed, for example, in the hoisting apparatus for large electric shovels or dragline applications in mining operations.
The present invention relates to a design for a wire rope with an improved strength over time and a longer useful life as a result of decreased wear, metal fatigue and corrosion compared to prior designs for wire ropes. The improved useful life resulting from the design of the present invention results in significantly reduced maintenance costs associated with wire rope replacement in, for example, electric shovel or dragline operations. The longer useful life of the present invention also has the advantage of keeping large, highly capital-intensive pieces of machinery, such as the large mining equipment in which the wire rope of the present invention is utilized, in use by significantly extending the time between replacement or maintenance operations and thereby reducing the average number of maintenance intervals required for a given period to maintain or replace the wire rope. This keeps the large equipment (such as mining equipment) productive longer and for a greater percentage of time, as compared to equipment using prior wire rope designs which require more frequent downtime to maintain or replace the wire rope.
By reducing wire rope deterioration due to wear, metal fatigue, abrasion due to contamination, strand breakage, or corrosion over time, the present invention enhances the safety of the operation of the large equipment utilizing the wire rope. For example, the reduction of wire rope deterioration reduces unexpected or premature failure of the wire rope in operation of the heavy machinery, which failure could lead to accidents, injuries, or deaths.
Furthermore and in addition to the advantages discussed above, particular embodiments of the present invention have added safety features as compared to prior wire rope designs. In one particular embodiment, for example, an optically transparent or translucent elastomeric or polymeric material is utilized for encapsulation of the strands and core of the wire rope, permitting enhanced visual inspection of wire rope strands, core, and component wires to visually observe the surface conditions of the strands, core, or component wires in the wire rope in order to determine whether they are broken, worn, corroded, contaminated, or otherwise deteriorated, for example, so as to determine whether the wire rope is in compliance with appropriate standards for continued use (see, e.g., Occupational Safety and Health Administration (OSHA) standards §§1926.550 and 1926.602, or Federal Specification RR-W-410E, as applied to worn or broken cores or strands of wire ropes). In yet another embodiment of the present invention, visibility of the wire rope may be optically enhanced by encapsulating the wire rope in elastomeric or polymeric material having high visibility coloring or reflective qualities such as those provided in ANSI/ISEA 107-1999 for worker's apparel. Visibility of wire rope in mining or manufacturing operations is important, especially because the equipment utilizing the wire rope is often, and in some cases is typically, operated in dark conditions (for example, at night, or in dark or confined spaces such as excavations or mines). Visibility of the wire rope is important both for purposes of proper operation of the heavy machinery and for safety reasons.
2. Brief Description of the Related Art
Prior designs for wire rope, when exposed to heavy duty applications such as mining or heavy construction operations, have shown a propensity for requiring relatively frequent maintenance, removal, and replacement. This frequent maintenance and replacement activity results in a number of expenses and other difficulties suffered by the user of the wire rope in such applications, including but not limited to: (1) the more frequent replacement cost of the wire rope itself; (2) the frequent (often daily) required lubrication of certain prior art wire ropes; (3) the more frequent downtime of a large piece of capital equipment using the wire rope, such as in electric shovel or dragline applications in mining operations, resulting in decreased productivity (the replacement of wire rope in a large electric shovel, for example, typically takes 5 to 8 hours, during which the shovel is idle and no production takes place); and (4) the large equipment utilizing the wire rope is not infrequently utilized in a remote or relatively inaccessible location, making maintenance and replacement more time-consuming and difficult, and generally more expensive.
It has consequently long been the goal of wire rope designers, manufacturers, and users to extend significantly the useful life of wire rope. This has become especially true as the applications in which wire ropes are used have become more rigorous. For example, in mining operations, large electric shovels used in the 1970s could typically lift up to 25 tons per shovelful, and utilized a wire rope having 1.5 to 1.75 inch diameters. Today, large electric shovels used in mining typically lift up to 100 tons per shovelful and often utilize wire ropes having 2.75 inch diameters. The larger diameter/heavier capacity wire rope, of course, is more expensive, and hence more expensive to replace or maintain. Thus, a wire rope having an extended useful life is more essential than before.
Wire rope designers, manufacturers, and users have utilized a number of approaches to attempt to extend the useful life of wire ropes. One technique for extending the useful life of wire rope is lubrication of the wire rope, with, for example, petrolatum. Lubrication was thought to extend the life of the wire rope by a number of mechanisms. First, lubrication would diminish the friction that would otherwise occur as a result of direct metal strand-to-strand or strand-to-core contact, such as would occur, for example, when a wire rope is bent. Lubrication would diminish strand or core wear, metal fatigue, and breakage caused by such contact within the wire rope. Second, lubrication of the metal strands and core would help diminish corrosion of the metal strands and core by helping to seal out corrosive elements, such as moisture, oxygen, or other corrosive elements or contaminants in heavy duty applications.
While lubrication proved somewhat useful, it had shortcomings. Lubrication would, over time, and over a relatively short time in some heavy duty applications, begin to wash or wear away from critical areas of the wire rope. This occurred for several reasons. First, in heavy duty applications where there was considerable rubbing between the metal strands and/or between the strands and the core of the wire rope, lubricant would eventually simply rub away. Second, because some petroleum lubricants such as petrolatum have a comparatively low melting point (petrolatum, for example, melts at about 97 to 140 degrees Fahrenheit, 36.11 to 60.00 degrees Celsius), friction within the wire rope, for example, when strands or the core rub during bending of the wire rope under loading, would heat the metal within the wire rope above the melting point of the lubricant, and the lubricant would melt and simply wash away, leaving a dry, uncoated wire rope, subject to frictional contact and corrosion, and hence, wear, deterioration, and eventual breakage. To compound matters further, the portion of the wire rope where the lubricant would rub or wash away earliest was often the same portion that was encountering some of the heaviest frictional contact during use. Thus, frequent re-lubrication was required to maintain and extend the life of the wire rope—indeed, in some heavy duty mining operations, the entire wire rope was required to be re-lubricated on a daily basis, adding significantly to maintenance and replacement costs, but even such frequent procedures would not prevent the wire rope from losing critical lubricant shortly thereafter.
A further problem with lubrication was that it would cause contaminants to stick to it. Some contaminants, such as abrasives or corrosive agents, when they adhered to the lubricant that was in turn coating the wire rope, would have a detrimental effect on the useful life of the wire rope, by causing wear or corrosion in the wire rope.
Another technique utilized for extending the useful life of wire rope was to impregnate the wire rope with thermoplastic. The goal of this technique was once again to avoid or minimize direct metal strand-to-strand contact, for example, when the wire rope was bent, diminishing strand or core wear breakage and thereby improving fatigue life. A further goal of this plastic coating resulting from thermoplastic impregnation was to seal the surface of the wire rope to inhibit corrosion resulting from exposure to moisture, oxygen, or other abrasive or corrosive elements found in heavy duty applications such as mining.
Once again, however, the results of such a wire rope treatment were not entirely satisfactory. Plastic impregnation occurred at elevated temperatures, therefore lubrication of the wire rope ordinarily needed to be avoided—the high temperatures required for thermoplastic impregnation resulted in a gas created by the lubricant, making plastic impregnation virtually impossible if the wire rope was previously lubricated. As a result, it became standard procedure to clean and de-grease the metal strands prior to plastic impregnation, and, consequently, the advantages of using lubricant on wire ropes were lost. (See, e.g., U.S. Pat. No. 3,824,777 at col. 1, lines 11-17).
Another issue arising with plastic impregnation was the inability to achieve even separation of the various component strands that made up the wire rope (see, e.g., U.S. Pat. No. 3,824,777 at col. 1, lines 17-21). During the course of fabrication of the wire rope, some of the strands and/or the core would move closer to one another than was called upon by the wire rope's design, and would sometimes even be in contact with one another at some locations, while other strands would have excess separation, making uniform plastic impregnation difficult, with the result being that the goals sought by such a process would not be fully achieved, and the useful life of the wire rope could not be extended as far as had been hoped.
To address these issues, U.S. Pat. No. 3,824,777 proposed a method of making a lubricated plastic impregnated wire rope. Heavy lubricant, such as petrolatum or asphalt based lubricant, was applied to component wires as the wires were formed into strands (the petrolatum would be applied cold, while the asphalt based lubricant would be applied hot). The lubricated wire was then preheated to about 100 degrees to 275 degrees Fahrenheit (37.78 to 135.00 degrees Celsius) and preferably 120 degrees to 160 degrees Fahrenheit (48.89 to 71.11 degrees Celsius), a temperature range in which the lubricants would not turn to gas during the later step of plastic impregnation. The lubricated wire rope was then kept at a balanced strand separation with a “strand gap controller” while being impregnated by an extruded thermoplastic at about 2,000 pounds per square inch (13.79×106 pascals) to 4,000 pounds per square inch (27.58×106 pascals) into the interstices of the rope.
One adverse result of making a wire rope as described in U.S. Pat. No. 3,824,777, however, was that impregnation of the wire rope with thermoplastic at high pressures via the extrusion process resulted in the plastic constraining the interior movement of the strands of the wire rope. In other words, because of the plastic impregnation, the strands of the wire rope could not readily move relative to each other. Such relative movement is desirable, and indeed necessary, for example, when the wire rope bends, especially under heavy loading. Constraining the strands and restricting their relative movement caused the strands to be strained and fatigued, resulting in premature failure of the strands, and thus the wire rope.
Another issue arising with wire rope made according to U.S. Pat. No. 3,824,777 was loss of lubricant resulting in sections having dry wire rope. While the manufacturing process for such wire rope was below the boiling points of the lubricants, and avoided the problem of applying plastic to wire when the lubricant was turned to a gas, preheating of the wire was often above the melting point of the lubricants, which resulted in much of the lubricant washing off during the plastic impregnation step of the process. The relatively high pressures utilized for plastic impregnation further contributed to lubricant wash off.
The wire rope made as set forth above, while accomplishing some desirable goals, nevertheless had significant shortcomings in terms of improved useful life. For example, it was found that wire ropes made as set forth above had a typical useful life, when used in heavy duty hoisting operations in the 1970s, of 423 to 776 hours, meaning that the wire rope required replacement approximately every 21.1 to 38.8 days of normal operations. Moreover, by the mid-1980s, larger electric shovels, with scoops having a capacity of 65 tons, were being introduced, for example, in mining operations, adding further impetus to the desire for an improved wire rope.
U.S. Pat. Nos. 4,509,319 and 6,360,522 attempted to address some of the shortcomings of prior wire ropes by introducing plastic inserts into the lubricated wire rope design. In these designs, strips of plastic filler material were inserted into the interstices between the central independent wire rope core and the individual outer strands of the wire rope, before closing and helically twisting the combined metal strands and plastic filler strips to form the finished wire rope. Different variations of plastic strips were employed. For example, U.S. Pat. No. 4,509,319 utilized reinforcing cores in the plastic filler elements, while U.S. Pat. No. 6,360,522 utilized different shaped plastic filler elements that sought to reduce vibration of the wire rope, wherein the plastic filler material had a bi-directionally oriented molecular structure to provide relatively higher tensile strength and high elasticity.
As the conditions encountered by wire rope applications continued to increase in terms of difficulty, further improvements in wire rope design became even more desirable. For example, in approximately 1998, 100 ton capacity large electric shovels were introduced into mining operations. Wire rope of the type disclosed in U.S. Pat. No. 6,360,522 was used on this type of shovel, and was observed to have an average useful life of approximately 920 hours. These large shovels were being typically operated three shifts per day, seven days per week (with the exception of downtime for meals, repairs, or breaks), or approximately an average of 20 hours per day for 360 days per year. Consequently, each set of wire ropes used in the hoisting apparatus for these large electric shovels would be required to be changed, on average, approximately 7.83 times per year. As stated previously, replacement of a set of wire ropes required approximately 5 to 8 hours, during which the large electric shovel was idle and thus unproductive, resulting in a substantial cost to the operator of the shovel.
Inspection of the spent wire-ropes made according to U.S. Pat. Nos. 4,509,319 and 6,360,522 showed that while wash off of the lubricant was reduced compared to some prior art wire ropes, it was not eliminated, because the surfaces of the strands and central core were partially exposed, permitting loss of lubricant, especially in stressful bending under heavy loads. Partial exposure of the metal strands in these wire rope designs also permitted contamination by moisture, abrasives, corrosive agents, and oxygen, and was believed to be another cause of premature deterioration and failure of the wire rope. It was also true that strand-to-strand and strand-to-core metal-to-metal contact, while reduced in these designs, was still significantly present, and was believed to be a factor in the reduction in useful life and ultimate failure of wire ropes made in these manners.
In view of the fact that large 100 ton electric shovels within which the wire ropes are utilized are becoming more prevalent, especially in mining operations, it was desirable to find a way to improve upon the design of the wire ropes to increase, in an economical manner, the useful life of the wire ropes.